Banding is a term used to define alternate streaks of light and dark areas (or bands) that are produced on an output sheet in a laser printer. These bands are produced as a result of non-constant placement of print scan lines. Dark bands are produced when the scan lines are closer together and lighter bands are produced when the scan lines are farther apart.
In a laser printer, pitch variation between scan lines can occur as the result of a number of system errors. The major cause is speed variations in the moving photoconductor. Those speed variations occur due to imperfections in the mechanical parts that comprise the photoconductor drive system. These imperfections may include out-of-round parts, gear inaccuracies, bearing runouts, mechanical coupling errors, etc. Structure vibration states which result from mechanical resonances can also cause relative motion between a laser scanner and the photoconductor and result in a banding affect on output sheets.
In general, banding is not apparent to the user when documents contain only text. However, because the resolution available from laser printers has seen substantial improvements, such printers are now employed to print full grey scale graphics images. Banding is clearly visible in such images.
When banding is apparent the extent of misalignment may exceed several scan line widths. In a monochrome printer, scan line misalignment is usually less than plus or minus up to one scan line width. For a color printer, banding is aggravated by misregistration of color planes in addition to scan line misalignment. A color printer has several color planes, for example, black, yellow, cyan, and magenta. Misregistration between planes can be several scan line widths.
The prior art has attempted to solve the banding problem by specifying more accurate (and more expensive) parts for the laser printer. Vibration problems have been reduced through provision of stiffer structures that are usually heavier and more expensive. Further, increasingly more sophisticated (and expensive) speed controls for drive motors have been implemented so as to assure relatively constant photoconductor speeds. Nevertheless, for laser printers that employ photoconductive belts, such improvements still do not eliminate banding. This is due to the fact that the belts themselves evidence some stretch and positional modification when subjected to impulse loading. A feedback system which attempts to correct for mechanical movements of the photoconductor and/or drive system cannot respond fast enough to totally overcome the banding effect. While high bandwidth servo systems approach a solution to the problem, they are still confronted with the problem of altering speed and/or position of mechanical elements which have both inertia and momentum that tend to negate the immediate correction actions. Such mechanical elements also exhibit a "resiliency" that tends to limit the responsiveness of the system to high frequency correction actions.
Accordingly, it is an object of this invention to provide an improved system for reduction of banding in a laser printer.
It is a further object of this invention to provide a banding reduction system in a laser printer wherein few additional mechanical parts are required.
It is yet another object of this invention to provide a banding reduction system for a laser printer wherein the banding reduction action is not dependent upon a high bandwidth servo system for correcting printer mechanical movements.